Described below is a device for selectively detecting gas components or a concentration of a gas component in a gas to be analyzed, a method for operating such a device and a system for the selective detection of at least two gas components or the concentrations of at least two gas components in a gas to be analyzed.
The detection of gases in the environment, in particular within closed spaces, has acquired a high degree of importance. If the concentration of gas components, for example, carbon dioxide, carbon monoxide, methane or water vapor is known, it is possible to warn of dangers in good time and therefrom to draw conclusions regarding, for example, the ventilation of spaces. Such concepts are gaining importance for the design of buildings because they offer great potential for energy saving and for more comfortable and resource-saving accommodation and working.
For selective and targeted detection of gas components or a concentration of a gas component in a gas to be analyzed, sensor elements—hereinafter called gas sensors—are used which are able to detect the concentration of one or more gases. Of decisive importance with gas sensors of this type, apart from the size thereof, is the energy consumption thereof because such sensor elements can often be operated self-sufficiently with regard to energy, that is, the energy therefor is to be drawn from the kinetic energy, heat energy and/or radiant energy of the surroundings.
A large number of measuring principles, for example, resistive, capacitive, thermal, amperometric, gravimetric, biochemical or optical measurements are known for gas sensors. Optical gas sensors are often based on the principle of absorption measurement, that is, on the fact that the gases absorb light in the infrared region in specific frequency ranges or at specific wavelengths. This is used in that light from a source is emitted at a specific wavelength along a test path, wherein the attenuation of the light due to absorption is subsequently evaluated by a sensor, for example, photometrically or thermoelectrically. In order to minimize transverse effects, a second reference path is often used, for example, in the form of a reference cell in which the absorption of the gas components to be detected is not expected.
A disadvantage of gas sensors of this type, however, is that in order to realize suitable detection sensitivity levels, very large spatial dimensions (several cm, up to several tens of cm) are required.
An alternative form of gas detection via optical absorption is possible by the use of photonic crystals. Photonic crystals are periodically structured dielectric materials which are the optical analogue of semiconductor crystals and therefore enable the production of integrated photonic circuits. Photonic crystals can be classified according to the dimensionality thereof. A distinction is therefore made between one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) photonic crystals, depending on the number of spatial directions having a periodic refractive index. Known photonic crystals are made of structured semiconductors, glasses or polymers.
DE 10 2005 008 077 A1 discloses a device for analyzing the qualitative and/or quantitative composition of fluids by a thermal radiator operating in the infrared spectral region. The radiator has a photonic crystal and generates the radiation through local temperature changes in a partial region of the photonic crystal. For this purpose, magnetic and/or electrically conductive material is introduced into the pores of the photonic crystal or the pores are coated with magnetic and/or electrically conductive material. The local temperature change is then generated by inductive and/or resistive heating of the partial region of the photonic crystal. The photonic crystal is configured such that the radiation emitted by the radiator is passed on only for a defined narrow wavelength range. For the detection of the gas, a device for regulating and/or measuring a heating output of the radiator is provided, wherein a radiator temperature is measurable at a fixed heating output or the radiator is adjusted to a constant radiation temperature and the heating power required therefor can be determined.
Silicon serves therein as the base material for the photonic crystal. However, this material has a very poor reflection factor, which significantly restricts the quality of the resonator. The detection results achievable therewith are not always satisfactory. Furthermore, the filling or coating of the pores with a magnetic and/or electrically conductive material represents a further operation in the manufacturing of the gas sensor, and this is associated with additional effort and cost. Filling the pores also leads thereto that the crystal structure can only enter at the end face thereof into an interaction with the gas to be analyzed, thereby severely reducing the sensitivity of the gas sensor. This disadvantage can be circumvented by the coating of the pores. However, the coating has the disadvantage that, for manufacturing technical reasons, the diameter and the spacing of the static silicon columns must not undershoot a particular minimum dimension, with the result that severe limits are placed on the miniaturization of the gas sensor.